1. Field of the Invention
The present invention relates to a printing mechanism and, more particularly, to a printing mechanism for printing letters using a printing head which includes printing type elements.
2. Description of the Prior Art
An impact-type printing mechanism is conventionally known. In this printing mechanism, a daisy-type printing type wheel having a number of radially disposed spokes, with the top of each spoke having a printing type, is rotated at high speed. When a selected printing type reaches a predetermined printing position, a printing hammer is activated synchronously therewith. The printing type is struck on a printing paper sheet, such as a recording medium, placed on a platen and thus printing is performed.
The printing mechanism of this type can perform high speed printing and produces clearly printed letters. However, the printing type wheel must be frequently replaced due to the short service life thereof. Further, a number of printing type wheels must be interchanged in accordance with the various type faces, resulting in inconvenience. Therefore, easy interchange of the printing type wheels is greatly desired.
FIGS. 1 and 2 show an example of a conventional printing mechanism of this type. A printing type wheel 2 has a number of radially disposed spokes, with the top of each spoke having a printing type portion 2bThe spokes are adapted to oppose a platen 1. The printing type wheel 2 is fitted on an output shaft 3a of an electric motor 3 through a boss 2a. A pin is disposed at the side surface of the motor 3. A spring 6 is hooked across the pin 4 and a pin 5 which is disposed at the stationary section of the printer. The motor 3 is pivotal about a shaft 7.
Referring to FIG. 1, the pin 4 is located to the left of the shaft 7 and on the side of the platen 1. The tension is applied on the spring 6 so as to pivot the motor 3 in the counterclockwise direction in the figure. The motor 3 is in contact with a stopper 8 in FIG. 1. Thus, the printing mode is initiated and printing with a hammer 9 is performed. However, in order to interchange the currently mounted printing type wheel 2 with another, the motor 3 must be pivoted about the shaft 7 in the clockwise direction as shown in FIG. 2. In this position, the pin 4 is located above the shaft 7 and a line connecting the pins 4 and 5 is located outside the shaft 7 with respect to the platen 1. Thus a pivotal force in the clockwise direction is applied to the motor 3. As shown in FIG. 2, the printing type wheel is widely separated from the platen 1 and the printing type wheel 2 faces substantially upward.
The structure of the printing mechanism described above is very simple and convenient. However, when the printing type wheel 2 is pivoted, a large space for the pivotal movement is required and a dead space is formed in the printer. When a structure is adopted wherein an ink ribbon is led between the platen 1 and the printing type wheel 2, the arrangement of the component parts is restricted. Further, when the stopper 8 is slightly misaligned, the relative positions of the platen 1 and the printing type wheel 2 and an angle formed therebetween tend to change greatly. In the printing mode, the spring 6 must provide a considerably strong tension, resulting in inconvenience in the operation by the operator.
As shown in FIG. 3, a structure has been proposed in order to decrease the space needed for the pivotal movement. Referring to FIG. 3, since the fulcrum is located above the pin 5 of FIG. 2 and behind the motor 3, the printing type wheel 2 can be pivoted outside the printer and the saved space may be used for other purposes.
However, when this structure is adopted, the boss 2a, the printing type portion 2b of the spoke and the like may come in contact with a contamination preventing plate (not shown) disposed between the platen 1 and the printing type wheel 2 and other component parts when the printing type wheel 2 is to be interchanged.
Further, in the conventional printing mechanisms of this type, the distance between the printing type wheel and the platen is restricted to 2 to 3 mm so as to improve the durability of the printing type wheel. Therefore, when a thick printing paper sheet is used or when a plurality of copies are required, the printing operation becomes cumbersome.